The fabrication of electronic devices such as transistors or surface acoustic wave transducers often requires the deposition of an aluminum film on a substrate. The formation of said films on substrates which are stable at high temperatures such as sapphire, quartz, silicon, etc. is known. Generally, the methods employ temperatures in excess of 250.degree. C. Optionally, known methods may incorporate a low temperature deposition process, i.e., temperatures in excess of 150.degree. C., but with an initial high temperature step to provide an oxide layer which will tightly adhere a subsequently deposited aluminum film to the substrate.
When the electronic device incorporates a piezoelectric compound, the compound is deposited on an aluminum film which has been previously deposited on a substrate which is stable at high temperatures. Direct deposition of an adherent pinhole free aluminum film on a piezoelectric and/or pyroelectric substrate is extremely difficult due to the nature of the substrate.
A piezoelectric substrate is defined as a substrate which exhibits the formation of an electrical current when pressure is exerted on the substrate. The deposition of an aluminum film on a piezoelectric substrate can exert sufficient pressure to electrically charge the surface of the substrate and create many problems with the attempted application of a pinhole free aluminum film.
A pyroelectric substrate is defined as a substrate which undergoes thermal deformation with an accompanying change in electronic properties upon heating. A pyroelectric material is a sub-group of piezoelectricity as taught by Cady in Piezoelectricity (1946), McGraw Hill. Aluminum films deposited on the pyroelectric surface, although tending to appear smooth, may contain pinholes which appear by blowing a jet of gas on the surface of said substrate, or when the deposited aluminum film is viewed under high power optical magnification.
A pyroelectric and piezoelectric substrate is a substrate which exhibits both of the above properties. Thus, a method which can deposit adherent pinhole free films onto pyroelectric and/or piezoelectric substrates would be highly desirable.